1. Field of the Invention
The present invention relates to a process for producing rubber-containing graft copolymer particles having excellent powder properties and which can be used for improving the impact resistance of rigid resins, such as polyvinyl chlorides, polystyrenes, polymethyl methacrylates, and acrylonitrile-styrene copolymers, and other resins known as "engineering plastics", such as polycarbonates, PET, PBT, and polyacetals.
2. Description of the Related Art
It is well known to blend a rubber-containing graft copolymer with rigid resins, such as polyvinyl chlorides, polystyrenes, polymethyl methacrylates, and acrylonitrile-styrene copolymers, and other resins called "engineering plastics", such as polycarbonates, PET, PBT, and polyacetals to improve the impact resistance of these resins. The rubber-containing graft copolymer is obtained, usually as particles or powders, generally by subjecting a latex prepared by emulsion polymerization to coagulation with an acid, a salt, or the like and then dehydrating and drying the coagulated particles.
However, the particles are likely to give rise to such problems that the particles will cause blocking during the storage thereof and that transportation lines of the particles will be clogged with the particles due to their poor fluidity. Further, when the rubber content in the rubber-containing graft copolymer is increased to sufficiently improve the impact resistance of the resin with which the graft copolymer is blended, the problems mentioned above will inconveniently become more serious.
Accordingly, with the recent trend toward automation in measuring the amounts of powders and of the oversizing of transportation lines, improvements in the powder properties such as blocking resistance and powder fluidity of the graft copolymers are urgently required.
Reflecting this situation, various processes have been investigated to improve the powder properties of rubber-containing graft copolymer particles. For instance, there are known a process in which finely powdered silica, sodium carbonate, calcium carbonate, titanium dioxide, or one of various metallic soaps is added to the powder particles of a rubber-containing graft copolymer, and a process as disclosed in Japanese Unexamined Patent Publication No. 64-26644 in which a lubricant is added to such particles or powders.
These processes, however, are accompanied by serious problems in that the improvement in the powder properties is insufficient, or that while a satisfactory improvement in the powder properties is observed, the effect of the rubber-containing graft copolymer on the improvement of the impact resistance is decreased, and the processability, transparency, and thermal stability of powder blends or final products thereof become poor.
Alternatively, a process has been disclosed in Japanese Examined Patent Publication No. 58-48584 (and corresponding British Patent No. 2,044,779) in which a graft copolymer having a low rubber content used for improving the powder properties is mixed with another graft copolymer having a high rubber content in a slurry state.
According to this process, however, the effect of the polymer mixture on the improvement of powder properties remains insufficient, although the physical properties of the graft copolymer having a high rubber content will not be adversely affected by the mixing, since the graft copolymer used for improving the powder properties contains a rubber component.
Further, the Japanese Examined Patent Publication No. 58-48584 (and corresponding British Patent No. 2,044,779) have disclosed, in their Comparative Example 3, a specific example where a polyvinyl chloride resin is blended with a modifier for improving an impact resistance (hereinafter referred to as impact modifier) prepared by mixing, in a slurry state, with a rubber-containing graft copolymer, a rigid non-elastic, two-stages polymer which contains no rubber component and is prepared by polymerizing a mixture of styrene and methyl methacrylate at a first stage and polymerizing methyl methacrylate at a second stage. However, the specific example shows that when such a two-stages polymer is mixed in a slurry state with a rubber-containing graft copolymer to form a modified rubber-containing graft polymer, many fish eyes are unfavorably produced when the modified graft polymer is blended with a polyvinyl chloride resin, and the level of impact resistance of the polyvinyl chloride resin to be expected will be lowered, indicating that the effect of the mixed rubber-containing graft copolymers on the improvement of the impact resistance of a resin to which the graft copolymers are blended is insufficient.
In addition, Japanese Unexamined Patent Publication No. 58-1742 (and corresponding U.S. Pat. No. 4,463,131) have disclosed that a rigid non-elastic polymer in a slurry state is mixed with a rubber-containing graft copolymer. The publications also have specifically disclosed the use of a copolymer having a molecular weight of 1,000,000 and comprised of methyl methacrylate component and ethyl acrylate component at a weight ratio of from 99:1 to 80:20. As long as such a copolymer is used, however, sometimes problems arise, similar to those described in Comparative Example 3 of the Japanese Examined Patent Publication No. 58-48584 mentioned above, that when the modified rubber-containing graft copolymer is blended with a polyvinyl chloride resin, fish eyes are unfavorably produced and that the impact resistance is lowered.
Further, Japanese Examined Patent Publication No. 61-57341 (and corresponding U.S. Pat. No. 4,097,553) have disclosed that a rigid non-elastic polymer is mixed with an acrylic rubber-containing graft copolymer in a slurry state to form a modified acrylic rubber-containing graft copolymer. As in the process mentioned above, however, this process will have defects such as fish eyes being produced when the modified graft copolymer is blended with a polyvinyl chloride resin, and the impact resistance being lowered.
As described above, it is a current situation that a process has not been found whereby the powder properties can be sufficiently improved without producing fish eyes and without impairing the impact strength, transparency, processability or thermal stability when a rubber-containing graft copolymer is blended as an impact modifier with a resin to which the graft copolymer is intended to blend.